两个会话分别只执行一个SQL会不会形成死锁

来源：恒创科技编辑：恒创科技编辑部

2023-12-24 14:38:59

这篇文章给大家介绍两个会话分别只执行一个SQL会不会形成死锁，内容非常详细，感兴趣的小伙伴们可以参考借鉴，希望对大家能有所帮助。

1，问题
   两个会话分别只执行一个SQL，可能形成死锁吗？

2，测试设想
   对于一个大表（比如100万条记录），两个会话分别从一个大表的两端（头、尾）更新，就可能形成互相等待对方已占有资源的情况，从而形成死锁。

3，测试
3.1测试版本：
Oracle Database 11g Enterprise Edition Release 11.2.0.3.0 - 64bit Production
PL/SQL Release 11.2.0.3.0 - Production
CORE    11.2.0.3.0      Production
TNS for IBM/AIX RISC System/6000: Version 11.2.0.3.0 - Production
NLSRTL Version 11.2.0.3.0 - Production

3.2 创建测试用表
create table tmp_x(x int,y int);

插入数据：
insert into tmp_x
select rownum,rownum
from dual
connect by level<1e6+1;

创建索引：
create index idx_tmp_x_x on tmp_x(x);

收集统计信息：
begin
dbms_stats.gather_table_stats(user,'TMP_X');
end;
/

3.3测试
--会话1

查看执行计划：
explain plan for update /*+ index_asc(t idx_tmp_x_x) */ tmp_x t set y=1 where x>1;
select * from table(dbms_xplan.display(null,null,'Advanced'));
Plan hash value: 4167283686
---------------------------------------------------------------------------------
| Id | Operation         | Name        | Rows | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------------
|   0 | UPDATE STATEMENT |             |   999K| 9765K| 4340   (2)| 00:00:01 |
|   1 | UPDATE           | TMP_X       |       |       |            |          |
|* 2 |   INDEX RANGE SCAN| IDX_TMP_X_X |   999K| 9765K| 2254   (2)| 00:00:01 |
---------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------
   2 - access("X">1)
Column Projection Information (identified by operation id):
-----------------------------------------------------------
   2 - (upd=3; cmp=2) "T".ROWID[ROWID,10], "X"[NUMBER,22], "Y"[NUMBER,22]

执行SQL：
update /*+ index_asc(t idx_tmp_x_x) */ tmp_x t set y=1 where x>1;

--会话2

查看执行计划（使用提示index_desc，CARDINALITY）：
explain plan for update /*+ index_desc(t idx_tmp_x_x) CARDINALITY(t 1000) */ tmp_x t set y=2 where x<1e6;

select * from table(dbms_xplan.display(null,null,'Advanced'));
Plan hash value: 2352573976
--------------------------------------------------------------------------------------------
| Id | Operation                    | Name        | Rows | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------------------------
|   0 | UPDATE STATEMENT             |             | 1000 | 10000 | 4340   (2)| 00:00:01 |
|   1 | UPDATE                      | TMP_X       |       |       |            |          |
|* 2 |   INDEX RANGE SCAN DESCENDING| IDX_TMP_X_X | 1000 | 10000 | 2254   (2)| 00:00:01 |
-------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
   2 - access("X"<1e6)
       filter("X"<1e6)

执行SQL：
update /*+ index_desc(t idx_tmp_x_x) CARDINALITY(t 1000) */ tmp_x t set y=2 where x<1e6;

---会话3
查看会话1,2的等待事件：
select sid,event,blocking_session from v$session where sid in (1894,2324);

       SID EVENT                         BLOCKING_SESSION
---------- ----------------------------- ----------------
      1894 SQL*Net message from client
      2324 SQL*Net message from client

SQL> /
       SID EVENT                         BLOCKING_SESSION
---------- ----------------------------- ----------------
      1894 gc current request
      2324 db file sequential read

SQL> /
       SID EVENT                         BLOCKING_SESSION
---------- ----------------------------- ----------------
      1894 db file scattered read
      2324 gc current request

SQL> /
       SID EVENT                         BLOCKING_SESSION
---------- ----------------------------- ----------------
      1894 gc current multi block reques
      2324 db file scattered read

SQL> /
       SID EVENT                         BLOCKING_SESSION
---------- ----------------------------- ----------------
      1894 gc current request
      2324 gc current request

SQL> /
       SID EVENT                         BLOCKING_SESSION
---------- ----------------------------- ----------------
      1894 gc current request
      2324 db file sequential read

SQL> /
       SID EVENT                         BLOCKING_SESSION
---------- ----------------------------- ----------------
      1894 db file sequential read
      2324 db file scattered read

SQL> /
       SID EVENT                         BLOCKING_SESSION
---------- ----------------------------- ----------------
      1894 log buffer space
      2324 log buffer space

SQL> /
       SID EVENT                         BLOCKING_SESSION
---------- ----------------------------- ----------------
      1894 gc current request
      2324 gc current request

SQL> /
       SID EVENT                         BLOCKING_SESSION
---------- ----------------------------- ----------------
      1894 enq: TX - row lock contention
      2324 enq: TX - row lock contention

SQL> /
       SID EVENT                         BLOCKING_SESSION
---------- ----------------------------- ----------------
      1894 enq: TX - row lock contention             2324
      2324 enq: TX - row lock contention             1894

SQL> /
       SID EVENT                         BLOCKING_SESSION
---------- ----------------------------- ----------------
      1894 enq: TX - row lock contention             2324
      2324 enq: TX - row lock contention             1894

SQL> /
       SID EVENT                         BLOCKING_SESSION
---------- ----------------------------- ----------------
      1894 enq: TX - row lock contention             2324
      2324 enq: TX - row lock contention             1894

SQL> /
       SID EVENT                         BLOCKING_SESSION
---------- ----------------------------- ----------------
      1894 enq: TX - row lock contention             2324
      2324 enq: TX - row lock contention

SQL> /
       SID EVENT                         BLOCKING_SESSION
---------- ----------------------------- ----------------
      1894 enq: TX - row lock contention             2324
      2324 enq: TX - row lock contention

SQL> /
       SID EVENT                         BLOCKING_SESSION
---------- ----------------------------- ----------------
      1894 enq: TX - row lock contention
      2324 SQL*Net message from client

--会话1
update /*+ index_asc(t idx_tmp_x_x) */ tmp_x t set y=1 where x>1
                                       *
ERROR at line 1:
ORA-00060: deadlock detected while waiting for resource

3.4测试结论
两个会话分别只执行一个SQL，也可能形成死锁。